A 70-year-old man saw his physician for a routine health maintenance examination. On physical examination, there were no remarkable findings, but a stool sample was positive for occult blood. A colonoscopy was performed and showed a 5- cm sessile mass in the upper portion of the descending colon at 50 cm from the anal verge. The histologic appearance at low power of a biopsy specimen of the lesion is shown in the figure. The patient refused further work-up and treatment. Five years later, he sees his physician because of constipation, microcytic anemia, and a 5-kg weight loss over the past 6 months. On surgical exploration, there is a 7-cm mass encircling the descending colon. Which of the following neoplasms is he now most likely to have?